1. Field of the Invention
This invention relates generally to the field of quantum cryptography, and more particularly to a method and apparatus for synchronizing the emitter and the receiver in an autocompensating quantum cryptography system.
2. Description of the Related Art
Quantum cryptography (QC) is a technique that allows two remote parties, an emitter and a receiver, to exchange in a secure and private way a sequence of bits to be used as a cryptographic key. The bits are encoded on individual two-levels quantum systems, like for example single photons, and transmitted over a quantum channel, like for example an optical fiber. According to the laws of quantum physics, an eavesdropper will necessarily introduce perturbations in the sequence of bits shared by the emitter and the receiver.
After the exchange of a large number of bits, the two parties perform key distillation. In the course of this procedure, they check the perturbation level on a sample of the bit sequence, also know as quantum bit error rate (QBER), in order to assess the secrecy of the transmission. In principle, errors should be encountered only in the presence of an eavesdropper. In practice however, because of the imperfections of the apparatus, a non-zero error probability is always observed Provided this probability is not too large, it does not prevent the distillation of a secure key. These errors can indeed be corrected, before the two parties apply a so called privacy amplification algorithm that will reduce the information level of an eavesdropper.
Various QC systems have been built by research groups (see Nicolas Gisin, Grégoire Ribordy, Wolfgang Tittel, and Hugo Zbinden, “Quantum Cryptography”, Reviews of Modern Physics 74, pages 145-190 (2002) for a survey). In real systems, one of the main difficulties is to reach a small error probability, even in the absence of an eavesdropper The overall error probability is the addition of several components. One of these components is linked to the so-called “dark counts” level of the photon counting detectors used to register the single photons. A second probability is related to the intrinsic contrast of the optical arrangement. This contrast must be as good as possible, and should not depend on too many f actors so that it does not vary over time.
One class of systems, the autocompensating systems developed at the University of Geneva by Hugo Zbinden, Jean-Daniel Gautier, Nicolas Gisin, Bruno Huttner, Antoine Muller, and Wolfgang Tittel, published within the article “Interferometry with Faraday mirrors for quantum cryptography”, Electronics Letters 7, 586-588 (1997) and furthermore published as WO 98/10560, utilizes the phase of the photons to encode the value of the bits. Because of time multiplexing and polarization fluctuations compensation by a Faraday mirror, the contrast of these systems is intrinsically high and stable. They are thus very well suited for QC in optical fibers. Contrary to other arrangements, with auto-compensating systems, an intense light pulse is sent on the quantum channel by the receiver to the emitter, who then attenuates it to the quantum level, encodes the value of a bit and reflects it back to the receiver.
One difficulty is for the emitter to know when to encode the value of a bit. This is particularly true because in practice sequences of pulses are travelling in the fiber. The emitter must accurately know which bit value he encoded on which particular pulse. The solution used until now involved the detection of the classical pulse by the emitter, the generation of an electronic signal which is delayed by the appropriate time before triggering the modulator used to encode the value of the bit.
The first problem with this approach is that if the emitter fails to register one of the intense pulses, his sequence of bits will be shifted with respected to that of the receiver.
A second problem comes from the fact that in autocompensating systems, intense pulses travel from the receiver to the emitter, while faint pulses travel back from the emitter to the receiver. In order to keep the error probability low, it is essential to discriminate between the faint pulses reflected by the emitter and the photon backscattered from intense pulses (Rayleigh back-scattering).
One possibility, proposed by Grégoire Ribordy, Jean-Daniel Gautier, Nicolas Gisin, Olivier Guinnard and Hugo Zbinden, in “Automated ‘plug & play’ quantum key distribution”, Electronics Letters 34, 2116-2117 (1997), is to discriminate temporally between these photons. The emitter then has a storage system—a long optical fiber—placed behind the attenuator. The pulses are sent in the form of trains by the receiver. The length of a train is smaller or equal to twice the length of the storage line, so that the intersection of the pulses takes place behind the attenuator. In this case, the emitter's modulator is located after the storage line, which means that the delay between the detection of an intense pulse and its arrival at the phase modulator is large. In addition this delay is larger than the time separating two subsequent pulses in a train. The delay generator must thus allow the input of several pulses before the delayed output of the first pulse. This is difficult to achieve with electronic circuits. One can use an optical delay line, but this is costly and unpractical.